QKD involves establishing a key between a sender (“Alice”) and a receiver (“Bob”) by using either single-photons or weak (e.g., 0.1 photon on average) optical signals (pulses) called “qubits” or “quantum signals” transmitted over a “quantum channel.” Unlike classical cryptography whose security depends on computational impracticality, the security of quantum cryptography is based on the quantum mechanical principle that any measurement of a quantum system in an unknown state will modify its state. As a consequence, an eavesdropper (“Eve”) that attempts to intercept or otherwise measure the exchanged qubits will introduce errors that reveal her presence.
The general principles of quantum cryptography were first set forth by Bennett and Brassard in their article “Quantum Cryptography: Public key distribution and coin tossing,” Proceedings of the International Conference on Computers, Systems and Signal Processing, Bangalore, India, 1984, pp. 175-179 (IEEE, New York, 1984). Specific QKD systems are described in U.S. Pat. No. 5,307,410 to Bennett, and in the article by C. H. Bennett entitled “Quantum Cryptography Using Any Two Non-Orthogonal States”, Phys. Rev. Lett. 68 3121 (1992). The general process for performing QKD is described in the book by Bouwmeester et al., “The Physics of Quantum Information,” Springer-Verlag 2001, in Section 2.3, pages 27-33.
The above-mentioned patent and publication by Bennett each describe a QKD system wherein Alice randomly encodes the polarization or phase of single photons at one end of the system, and Bob randomly measures the polarization or phase of the photons at the other end of the system. The QKD system described in the Bennett 1992 paper is based on two optical fiber Mach-Zehnder interferometers (one at Alice and one at Bob). Respective parts of the interferometric system are accessible by Alice and Bob so that each can control the phase of the interferometer.
FIG. 1 is a schematic diagram of a prior art QKD system 10 based on those disclosed in U.S. Pat. No. 5,307,410 to Bennett (“the Bennett patent”) and U.S. Pat. No. 5,953,421 to Townsend (“The Townsend patent), which patents are incorporated herein by reference. QKD system 10 includes two QKD stations Bob and Alice. Not shown in FIG. 1 are controllers in Alice and Bob that control the operation of their respective elements, and that are in operable communication with each another to coordinate the operation of the QKD system as a whole.
Alice includes a laser source L1 and a first interferometer loop 12 formed from beamsplitters 11 and 13 connected by optical fiber sections 14 and 16 of different lengths. Optical fiber section 14 includes a modulator (polarization or phase) MA. Interferometer loop 12 is coupled to an optical fiber link FL, which is connected to a second interferometer loop 22 at Bob. Loop 22 is formed from beamsplitters 21 and 23 connected by optical fiber sections 24 and 26 of different lengths. A (polarization or phase) modulator MB is arranged in optical fiber section 24. Loop 22 is coupled to a detector unit 30 via an optical fiber section F3 optically coupled to beamsplitter 23. The detector unit 30 may include, for example, two single-photon detectors (SPDs) coupled to optical fiber section F3 by an optical coupler, such as illustrated and discussed in the Townsend patent. Detector unit 30 may also include a single SPD, such as illustrated and discussed in the Bennett patent.
In operation, laser source L1 generates a light pulse P0 that is divided into two pulses (signals) P1 and P2 by first interferometer loop 12. One of the pulses (say P1) is randomly modulated by modulator MA. The two pulses, which are now separated due to the different path lengths of the optical fiber sections 12 and 14, are attenuated so that they are weak (i.e., one or less photons per pulse on average). The photons then travel over fiber link FL to second interferometer loop 22.
At interferometer loop 22, each signal P1 and P2 is then split into two signals (P1 into P1a and P1b, and P2 into P2a and P2b). Two of the signals (say P1a and P2a) travel over optical fiber section 24, while the other two signals (say P1b and P2b) travel over optical fiber section 26. One of the signals traveling over optical fiber section 24 (say, P2a) is randomly modulated by modulator MB.
The second interferometer loop then combines the signals onto fiber section F3. If the two interferometer loops 12 and 22 have the same overall path length (e.g., the lengths of optical fiber sections 14 and 24 are the same, and the lengths of optical fiber sections 16 and 26), then the two signals that travel the same optical path length (say, pulses P2a and P2b) interfere to create a single interfered signal I. The other ancillary signals AS enter fiber section F3 separated from one another because they followed optical paths of different lengths.
The interfered signal I is then detected by detector unit 30 in a manner that reflects the polarization or phase modulation imparted to the interfered signal. The process is repeated to create a number of interfered signals I, which are detected and processed according to known QKD techniques to establish a secret key between Alice and Bob.
The use of interferometer loops formed from optical fibers or beam splitters to create multiple signals is standard in QKD systems. However, such arrangements tend to be lossy and are fairly complex because the loops have to be thermally stabilized. Further, there is a strict requirement for interferometer arm balancing. A laser L1 normally has narrow signals (for example, with full width at half maximum (FWHM) of approximately 100 ps), so the lengths of short-long arms should be balanced within an accuracy of hundreds of microns to obtain a good extinction ratio. Interfering signals (e.g. P2a and P2b) should overlap in the time domain. In manufacturing, this puts strict requirements on fiber splicing and system component selection.